Lithium ion secondary batteries have been widely used as batteries for mobile phones and electric vehicles, since lithium ion secondary batteries have high theoretical energy densities as compared to conventional secondary batteries. In conventional lithium ion secondary batteries, an organic solvent-based electrolytic solution that does not decompose even at a voltage of approximately 4 V is used in order to achieve a high energy density. As such an organic solvent, typically, ethylene carbonate, diethyl carbonate, etc. are mainly used (Patent Literature 1).
However, an organic solvent is generally combustible, and thus ensuring safety particularly in large-sized secondary batteries for vehicles and power storage is an important issue. In addition, the ionic conductivity of an organic solvent solution is very low as compared to that of an aqueous solution, so that rapid charging/discharging characteristics of the organic solvent solution are not sufficient, which is a problem. Meanwhile, in view of such a problem, research has been conducted for aqueous lithium ion secondary batteries in which an aqueous solution is used as an electrolytic solution. However, water theoretically decomposes at a voltage of 1.23 V, and thus an aqueous lithium ion secondary battery that stably operates even at a high voltage that is not less than this voltage has not been achieved.
Capacitors are power storage devices which store or release charge by adsorption or elimination of ions in an electrolytic solution with respect to an electrode surface. Capacitors are roughly categorized into: organic solvent-type capacitors in which an organic solvent and a quaternary ammonium salt or the like are used for an electrolytic solution; and aqueous capacitors in which water is used as the solvent of an electrolytic solution.
The amount of charge that is stored in a capacitor is represented as a product of capacitance and voltage. Regarding an aqueous capacitor, the upper limit of the voltage is limited due to the withstand voltage of water, and thus studies for increasing the capacitance are generally conducted in order to increase the amount of charge.
As a matter of fact, Patent Literature 2 discloses a technique of an aqueous capacitor to optimize the pore size and the specific surface area of activated carbon to be used for an electrode of a capacitor, thereby increasing capacitance.
However, since the withstand voltage of water is low as described above, putting an aqueous capacitor that stably operates at a high voltage to practical use has been difficult.